The phenotype, or physical manifestation of the genome, can take many forms based on what genes are passed down to an organism from its parent(s). In the case of sexually reproducing organisms such as humans, half of an organism’s DNA comes from the mother, and the other half from the father. In humans, each trait is determined by 2 alleles, or different versions of a gene, one of which comes from the mother and the other from the father. Some phenotypic traits are determined by more than 1 gene, like skin color, in which multiple genes play a part in determining the phenotype. There are 2 different kinds of alleles: dominant, and recessive. A dominant allele is the one that will be expressed if there are 2 dominant alleles present (homozygous) or 1 dominant and 1 recessive allele present (heterozygous). The recessive allele will only be expressed if it is homozygous recessive, which means 2 recessive alleles are present. Some genes alleles show incomplete dominance, where the dominant allele doesn’t entirely mask the recessive, and some show codominance, in which case both alleles are expressed.
A widow’s peak is the dominant phenotype for the hairline trait. Widow’s peak is the triangular shape shown here, formed by the center of the hairline. Since it is dominant, this man either has a WW or Ww genotype. However, since he is my dad and I do not have a widow’s peak, he must be heterozygous for the trait with a Ww genotype.
This lady has the recessive phenotype of a straight hairline, making her genotype ww, or homozygous recessive. This makes her alleles for the same trait different from his.
The slight bend in the thumb of the hand on the left is called hitchhiker’s thumb. Hitchhiker’s thumb is a recessive trait, so the hand on the left has a homozygous recessive genotype (ss) for this trait, while the hand on the right with a straight thumb may have a heterozygous or homozygous dominant genotype (Ss or SS).
There is some debate as to the genotype of different types of hair, but most agree that straight hair is recessive. Some debate that straight vs. curly is determined by a single gene, in which straight is recessive and curly/wavy is dominant. However, others say that hair is determined by many different genes.
According to the first theory, she could be heterozygous or homozygous dominant for wavy hair, or have multiple genes and alleles playing a role in determining her hair texture. Either way, she would have different alleles than the man with straight hair.
Tongue-rolling is definitely linked to genetics, but many scientists believe that it is not a simple rolling/not rolling gene. There is speculation that multiple genes are involved in this expression, but there is a direct link showing that parents that can tongue-roll are more likely to have kids that can roll their tongues. The man on the left does not express the tongue rolling phenotype while the woman on the left does, showing that they have different alleles for the same trait.
The person on the left is a male while the person on the right is a female. The man has one X chromosome and one Y chromosome while the woman has 2 X chromosomes. The trait is gender and the two have different alleles for it, making them opposite genders.